Sometimes, if you put walls or a foundation in at all it can mess up the basement even after you remove them. It will still say "Blocked by wall" or "Cannot place inside foundation," despite the fact that there is absolutely nothing above ground. The best bet is to not place either until all the stairs you want are installed. When I do my typical legacy, I put in all 4 basement levels before I build the house.
The easiest way I have found to do this is to first build a basement 1-2 squares larger than necessary to accommodate the stairs, remembering that lower levels will require enough space to accommodate them on the upper level as well. So if your desire is to install two flights of single wide stairs next to each other, the basement needs to be at least 4 squares wide on the upper level. Once you have all the basement levels installed, start at the bottom and work your way up. Once all the stairs are in, you can modify the size of the levels as you wish.
Another thing to consider about stairs (if you haven't already) is that a single wide section of stairs is a big time waster for your sims, especially when it is the only access to the basement. If two sims are going in the opposite direction, one must wait until the other has finished using them. This especially causes issues when you have to build basement stairs through a foundation (requiring a landing on the ground level and a set of sub stairs from the foundation to the landing). It can cause path routing issues where a sim becomes stuck, forcing you to manually move one out of the way so the other can move. The problem increases when one of the sims is a visitor, and you don't control one of them. This is why, in any house I build myself, you will never see a single wide staircase, unless there is another leading to the same level.
